Fix the ComplexTypeWithAttributeGroupExtension test on Linux and macOS

On Linux and macOS, the generator writes with LF (instead of CRLF) so the normalization Regex was not matching `\r\n`.

XmlSchemaClassGenerator.Tests/XmlTests.cs

@@ -1410,7 +1410,7 @@ public void BoolTest()
 
  private static void CompareOutput(string expected, string actual)
  {
- static string Normalize(string input) => Regex.Replace(input, @"[ \t]*\r\n", "\n");
+ static string Normalize(string input) => Regex.Replace(input, @"[ \t]*\r?\n", "\n");
  Assert.Equal(Normalize(expected), Normalize(actual));
  }